Among high-pressure discharge lamps, high-pressure mercury lamps in which mercury is filled as a light emitting substance has recently been attracting attention as light sources for liquid crystal projectors.
In a high-pressure mercury lamp, a pair of electrodes extends into a discharge space such that the tips of the electrodes face each other with a distance therebetween. The lamp keeps lighting by causing an arc discharge between the pair of electrodes. At the start of the lighting, the arc discharge does not immediately occur between the tips of the electrodes. Instead, first a discharge occurs at the base of an electrode in the discharge space (hereinafter called the “the electrode base part”). The discharge transfers along the inner surface of the discharge vessel forming the discharge space, from the electrode base part of one of the electrodes to the electrode base part of the other one of the electrodes (or the tip of the other one of the electrodes).
The discharge that occurs at the base of an electrode is hereinafter called “the base discharge”. The base discharge occurs because the temperature in the discharge space and the mercury vapor pressure between the tips of the electrodes are both low. After the base discharge occurs, the base of the electrode becomes an arc spot. The arc spot causes the material (tungsten) of the electrode to evaporate. The evaporated material attaches to and accumulates on the inner surface of the discharge vessel. The accumulation is called “blackening”. The blackening on the inner surface of the discharge vessel leads to a short life of the lamp due to reduction in the luminous flux maintenance factor.
Japanese Laid-Open Patent Application Publication No. 10-188896 is an example of prior art documents relating to the invention of the present application.
Liquid crystal projectors having such a high-pressure mercury lamp in the past were used mainly in school classrooms, conference rooms, and the like, but in recent years have become increasingly popular with ordinary households.
The liquid crystal projectors used principally in school classrooms and conference rooms are in use for a maximum of a few hours each day. The projectors used as TV displays or home theaters, on the other hand, are used continuously. Hence, it can be assumed that a period of use is incomparably longer than that of the conventional mode of use. Consequently, the life (e.g. 2000 hours) of the projectors that were used mainly in the school classroom and conference rooms is insufficient, and a life several times that of previous lamps is required.
The liquid crystal projectors for use in ordinary households are required to be small and light for portability and easy setup.